It is beneficial to have a workbook to reference in most cases. However, sometimes the questions are so general that an example is superfulous. More often, though, the person posing the question is working in a file that is so large and complex and contains mostly confidential data that it is just not practical to post it.
Yes, it would be nice. :)
The reason I commonly ask for sample data is because there are multiple ways to achieve the desired result in Tableau. The route depends on the data structure. Tableau does not generate data, Tableau connects to data, and that data structure you are connecting to sets the bounds of what can be done in Tableau. With the ability to transform data prior to Tableau, there are many possibilities, without, it is a constraint that needs to be known in order to offer a possible route. Sometimes transforming data for Tableau is more trouble than it is worth, and using calculations is the better route. Knowing what the raw data looks like is an important first step in working with data in Tableau. The first question I need to answer in order to work with data in Tableau is what combination of dimensions describes a distinct row?
If the person asking for help cannot create a few sample fake data records that represents their situation, it makes it more difficult to provide assistance. Providing sample data enables assistance. While not every question needs sample data to be answered, providing sample data makes it easier to help. The more complex the situation, the more beneficial sample data becomes.